As machine tools for drilling holes in a workpiece, there exist portable drilling devices having portability and a structure in which a body thereof can be secured to a workpiece by means of an attracting force of an electromagnet, a vise, or the like. Generally, drilling devices have a structure in which a drill is connected directly to a drive motor (hereinafter, referred to as “a motor” for short). A load acting on the drill significantly fluctuates according to a contact condition of the drill with the workpiece, a downward force applied to the drill, material properties of the workpiece, and so on. This fluctuation is transmitted directly to the motor. Therefore, in a case where a low-power motor is used, if an overload condition continues for a long period, the motor may burn out due to an overcurrent generated in the overload condition.
There is known a drilling device in which, for example, different first and second reference levels are defined for preventing overload of a motor. When a load current exceeds the first reference level, an alarm is generated. When the load current exceeds the second reference level, an alarm is generated and power supply to the motor is stopped. Such a drilling device is disclosed in, for example, Patent Document 1 below.
There is also known an electric drilling device in which a current flowing through a motor is interrupted when a load current exceeds a reference value, and power supply to the motor is automatically resumed after a lapse of a predetermined period after the load current is reduced to the reference value or less. Such an electric drilling device is disclosed in, for example, Patent Document 2 below.    Patent Document 1: Japanese Examined Patent Application Publication No. 62-6295    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-52914